This looks like a very cool project and I'll definitely play around with it.
But I would be remiss if I didn't also mention that my expectations on the performance and memory characteristics of such a project terrify me a little.
You'd be right to be terrified, it's not exactly zippy. It's a toy, but I'm releasing it because it's got a couple of nice features (friendly REPL, lazy evaluation) and it's more complete in some areas than other toy Schemes I've tried. Also I figured it might be a nice example implementation for people who know Ruby. If you have optimisation suggestions or links to stuff I should read on the subject I'd be delighted.
But I would be remiss if I didn't also mention that my expectations on the performance and memory characteristics of such a project terrify me a little.